Thermal neutron shield and method for making same



United States Patent 2,727,996 Patented Dec. 20, 1955 RMAL NEUTRGN SHIELD AND NETHUD F OR MAKING SAME 1 No Drawing. Application August 11, 1952,

Serial No. 303,704 I 23 Claims. (Cl. 250-108) The present invention relates in general to an improved shielding medium for providing protection against emanations of radioactivity, especially neutrons, and more particularly to such a shielding medium of universal applicability for improved shielding primarily against neutrons of thermal energy, especially those in the radiation emission spectra of nuclear reactors.

While radioactive emanations have long been known and used and have served many useful purposes, it has been appreciated that there are considerable physiological and biological hazards associated with the handling and use of radioactive materials. Furthermore, apparatus and equipment can often suffer deleterious eifects when exposed to various types of radiation. In recent years, with the greatly increased utilization of radiation and radioactive materials, and with the development of. sources of radiation of exceedingly high intensities, the problem of protection from radiation effects has become extremely important.

The conventional method of affording protection to personnel and equipment against radiation is the provision, between them and the source of the radiation, of a shield or barrier substantially opaque to the radioactive emanations. in the past, when radioactivity was utilized on a small scale and the intensities of the emanations were of a comparatively low value, radiation shielding was satisfactorily eflfected by providing a barrier of a sulficient quantity of any relatively dense material, it being well known that a sufiicient amount of matter of any kind will absorb almost every type of radiation. However, with the advent of the nuclear reactor, the cyclotron, and other devices and processes wherein radiation in enormous quantity and of great energy may be produced, the problem of shielding became a much more. complicated. matter. In such cases it is no longer satisfactory to employ. indiscrim'inately any type of relatively dense matter for shielding; rather, practical considerations make desirable the use of shields of minimum weight, volume, and cost. This is particularly true in the shielding of nuclear reactors. In stationary installations, reactors ordinarily assume relatively huge' proportions; and in mobile installations, where reactors may be employed for the generation of motive power in airplanes, ships and the like,fthe magnitudes of the weight and bulk of the shielding will often be the critical factors in'determining the practicability of the power unit. It is with the improved shielding of. such reactors that the present invention is primarily concerned.

The fundamental theory, details of construction, and

principles of operation, of nuclear reactors are now widely known in the art. For such details, specific reference is made to published papers, as for example: The Science and Engineering of Nuclear Power, vols. 1 and 2, edited by Clark Goodman, Addison-Wesley, 1949; Elementary Pile Theory, Soodak and Campbell, 1950," Wiley; First detailed description of the AEC research reactors, Atomics, vol. 6, No. 6, November-December, 1950, pages .4-22; and copending application, as: lSer. No. 568,904,

filed December 19, 1944, in the names of E. Fermi and L. Szilard, now Patent No. 2,708,656, May 17, 1955.

A typical reactor, for example, is constituted of a cubical core of graphite ca. 20 feet in each principal dimension, built up of stacked graphite bars, having a multiplicity of parallel horizontal channels passing completely therethrough, and having a multiplicity of masses of natural uranium metal disposed within such channels. The atomic ratio of carbon to uranium in the cube is of the order of 200, such that the average neutron energy in the system closely approaches that of the normal thermal energy of neutrons at the ambient temperature,

i. e., ca. 0.025 electron volt at room temperature (termed.

thermal neutrons). The cube has adjustably inserted therein. a plurality of control rods, comprising a strong neutron absorber, such as cadmium, whereby the fission rate may be appropriately regulated by adjustment of the extent of the rods withdrawal from the cube. Air, or other coolant, is continuously blown or drawn through the aforesaid channels, which are only partially occupied by the masses of uranium, to remove heat generated within the cube. In operation, the rods are withdrawn sufficiently that a self-sustaining chain fission reaction is achieved, and a constant fission rate and neutron flux level are maintained. During such operation, radiation of various types-principally neutrons, gamma rays, and alpha and beta particlesin enormous quantity and of energies ranging to exceedingly great intensity are continuously emanated in all directions from the operating cube. Among these, neutrons and gamma rays are, by far, the more diificult to attenuate and absorb, with such difiiculty increasing with increase in the energy of the neutrons and gammas. Representative of the spectrum of radiation ordinarily emanated from such a reactor are the data presentedin Table 1 following.

Table 1 [Typical nuclear reactor neutron {and gamma 2 radiation spectrum (approzzmateJl 1 For bare thermal reactor; graphite mod crate d; air cooled.

2 Alpha and beta radiation is relatively insignificant here, since in any probable reactor radiation spectrum, the amount of shielding necessary to attenuate neutrons and gammas to tolerance will be more than sufficieut for stopping virtually all alpha and betaradiatlon.

Million electron volts. 1

To restrain such radiation from dangerously escaping, it is conventional to encasethe core in a shield of massive material several feet thick, and spaced about one or two feet from the faces of the cube, normally so as to form a cooling-air chamber in which around and through the cube to remove heat generated therein. In the past, it had been common to use structural concrete to constitute the massive shielding. More recently, in the interest of decreasing the weight and volume of shielding needed, special concrete compositions of enhanced shielding efficiency have been devised. For example, such an improved shielding concrete is that described in copending application Ser. No. 79,522, filed March 3, 1949,, in the. name of Lyle B. Borst, for Concrete Shielding Composition, which composition comprises structural concrete having uniformly dispersed therein atoms of a dense, high-atomic-weight, metallic element, such'as ironor' lead. However, further improvement. in the overall eifi'ci'ency of reactor shielding is air is passed no simple matter. ent component substances varies greatly not only among the different types of radiation, but also among different energy levels of the same type of radiation. For instance, the efiicacy of elements in absorbing gamma rays generally increases with increase in atomic weight, while,

on the other hand, elements of the lowest atomic weights are generally the most effective in attenuating slow neutrons. For attenuating fast neutrons, though, elements of high atomic weight are the better. The difficulty is compounded by the fact that attenuation or absorption of one type of radiation is often accompanied by immediate emission by the shielding material of radiation of a different type. Such radiation, generated within the shield itself, is termed secondary radiation; most deleterious among these is the often-high-energied secondary gamma radiation which ordinarily attends the deceleration of neutrons (primarily fast neutrons) and absorption of neutrons (primarily slow, including thermaljneutrons) by most materials, especially those of high atomic weight. The problem of secondary radiation production becomes particularly acute when dealing with the wide variety of types and energies of radiation emitted by a nuclear reactor. Often, the materials which are the most effective in attenuating one type of radiation are just the materials which most adversely react with other types of radiation to produce secondary radiation. For example, metallic elements of reasonably high atomic weight, which, as noted before, are advantageous ly the more efiicient in fast neutron and gamma ray attenuation, are also particularly obnoxious materials in producing high-energied secondary gamma radiation upon interaction with neutrons. This high-energied secondary gamma production effect is so pronounced that it has been found that if it were attempted to shield an ordinary nuclear reactor with a reasonable thickness of iron or lead, the gamma flux leaving the shield would be greater than the entering gamma flux. Similarly, attempts to afford thinner shields by aggregating materials which are separately of the highest effectiveness in attenuating each species of reactor radiation alone, tend to result in a radiation spectrum, leaving the shield, which is still damaging and deadly, though different from that entering the shield. Likewise, special distribution of the components of a reactor shield has not proven generally profitable as an avenue of approach toward affording shields thinner than those described in aforesaid copending. application Ser. No. 79,522. For example, it would at first appear to be a promising approach toward mitigating the secondary gamma production difficulty, to displace all the iron or lead inclusions, desirable for efficient gamma absorption, to the outer layers of the concrete shielda region where the thermal neutron flux will have been grossly removed in the earlier mass of hydrogenous concrete-so as to largely avoid at least that secondary gamma production attending absorption of the slow neutrons in the incident spectrum; yet it is found that fast neutrons from the reactor course with much less restraint through the inner concrete mass and then upon reaching the iron or lead, are there both decelerated to thermal neutrons and thereupon efficiently absorbed in the iron or leadboth of which effects there are accompanied by high-energied secondary gamma production. These highly-dangerous secondary gammas, it is to be noted, are generated in the outer surface of the shield, wherefrom they escape, largely unchecked. Upon adding a further mass of gamma shielding beyond the iron or lead to reduce the dangerous secondary gamma radiation to tolerance, the purpose of providing thinner shielding wouldvbe defeated.

Despite the obstruction imposed by these difficulties, there has been an increasing desire that new, effective means be found for more efficient shielding of nuclear reactors, especially means directed primarily to mitigating and overcoming the secondary gamma ray -produc-,

The'shielding effectiveness of differ- I tion problem. It has been recognized too that for reasonable practicability, it is highly desirable, also, that such shielding means be mechanically self-supporting, able to be readily fabricated into diverse shapes and contours, adapted to be perforated easily to form various ports and ducts which conventionally penetrate reactor shields, and adapted to maintain disposition and integrity dependably in operation, without loss or displacement of essential components.

Accordingly, one object of the present invention is to provide an improved means for affording more eflicient shieldingless ponderous and bulky than conventional media-against radiation from nuclear reactors.

Another object is to provide an improved means for shielding against neutrons, particularly neutrons of thermal energy.

A further object is to provide such a means especially adapted to application to conventional nuclear reactor shielding for substantially mitigating the generation of secondary gamma radiation therein.

Still another object is to provide such a means in the form of a self-supporting, thin, metal sheet, which is able to be readily fabricated to diverse shapes, contours, and perforations.

Still a further object is to provide such a means which maintains its disposition and integrity during operation.

Yet another object is to provide such a means adapted to be manufactured in quantity as a standard, universal means for improved shielding for nuclear reactors, and for neutrons generally.

Yet a further object is to provide such a means of practical fiitness and suitability appropriate for mobile reactor application.

A complemental object is to provide improved methods for making such means.

Additional objects will become apparent hereinafter.

In accordance with the present invention, an improved means for shielding against neutrons, especially thermal neutrons, and for affording nuclear reactor shielding of enhanced efficiency, comprises, as an article of manufacture: a continuous matrix of a malleable metal; discontinuously and uniformly dispersed therein, a solid, refractory material substantially constituted of boron; and, containing said particle-bearing matrix, and substantially bonded metallurgically thereto, a sheathing of a malleable metal. In most versatile form, the article comprises about 0.5 to 1.0 gram of boron per square centimeter, which surface concentration is readily provided in a matrix of only a fraction of an inch thickness, sandwiched between paper-thin protective metal sheets. Despite its extreme thinness, it has been found that a sheet of the article of such boron content will consistently attenuate an incident thermal neutron flux by a factor of ca. 10 i. e., it permits only one ten-billionth of incident thermal neutrons. to pass through, and absorbs all the rest. In addition, particularly upon employing the preferred malleable metal component, aluminum, to constitute the continuous matrix and protective sheathing, the degree of secondary gamma production associated with such neutron absorption is so extraordinarily small as to be quite innocuous, and effectively negligible, from a practical shielding standpoint. Therefore, the most intense sources of pure thermal neutron flux encountered in current practice may be effectively shielded by simply encasing it in a thin metal envelope of applicants article. In more complex application to improving nuclear reactor shielding, it has been found highly effective to cover the inner face of a conventional massive structural concrete shield with a plate, of fractional inch in thickness, of the present article. Applicants plate serves to screen virtually all of the thermal neutrons from the spectrum of reactor-emitted radiation entering the shield, without serious high-energied secondary gamma ray production. By virtue of the substantial reduction not only in neutron flux, but, highly significantly, in the number of' secondary gammas generated in the shield, elimination of a substantial thicknessof ordinary concrete otherwise required at the outer surface'of' theshieldis afforded. For example, in the case of reactors of the typical type alluded to hereinb'efore, elimination of over two feet of structural concrete from the outer surface of the shield is generally afforded by application of applicants plate to the inner face. Since as a geometrical result, weight and bulk of an enveloping shield generally increases as the cube of the thickness of the shield, this elimination ordinarily represents an enormous improvement. Of particular advantage, the present article, being a thin but rigid metallic'plate,-may simply be bolted into place at the inner surface of a conventional concrete reactor shield, and for simple and easy fabrication it may readily be sheared, sawed, welded, punched, drilled, tapped, rolled, hot pressed, and the like, without damaging its integrity. Furthermore, while in nuclear reactor shielding application, considerable heat generation in the thin plate results from the absorption of virtually the entirety of the enormous slow neutron flux, yet theplate, beneficially being metallic, has a sutficiently high thermal conductivity to dissipate the heat continuously, without undue temperature rise. Moreover, being a continuously bonded and sheathed solid, the components of the plate are not inherently susceptible to shifting, settling, or leaking away, with the attendant risk of dam age and destruction of the entities to be protected. Having such beneficial attributes, the present medium clearly affords substantial practical advantages in radiation shielding. I p

Considering the constitution of the present medium in more detail, for serving as the specified refractory material constituted substantially of boron, high-meltingpoint mineral compounds of boron are preferred, while metallic boron is also satisfactory. It is highly desirable that the material contain a relatively high density of boron, and that other elements present in the compound do not engage in excessive secondary gamma ray production upon interaction with neutrons. 'Boron carbide (ll-1C), being eminently satisfactory in'these respects, is' the particularly preferred boron material; beneficially, its carbon content not only doesnt engage in pronounced high-energized secondary gamma production, but is an ehicient neutron moderant thereby serving to decelerate faster neutrons to the thermal energy range, Where they are more efiiciently absorbed by applicants plate. Boron oxide (B203) is also satisfactory, as are sodium tetraborate, boron phosphide, and boron sulfide, which, however, are constituted of anions which are somewhat less desirable. in view of the thinness of the plate, particle size of the boron material should be as small as practicable in order to effectively approach homogeneous distribution; the approximate range 20 to 100 mesh size has been found to be the convenient optimum.

For the continuous matrix of a malleable metal, aluminum is the particularly preferred material, in that it is notably malleable, and again, in addition, is a superior neutron moderant and does not generate excessive highenergied secondary gamma radiationupon interaction of neutrons. For the same reasons zirconium is also a highly satisfactory malleable metal for this purpose, although, having a melting point (3450 F.) much higher than aluminum, it is less convenient in fabrication operations. Nevertheless, this very high-melting characteristic renders zirconium the particularly preferred metal, for this purpose, for high-temperature shielding applications. In general, any metal that can be hot-worked is suitable, including: nickel, iron, copper, chromium, Wolfram, and lead. However, since these latter metals, especially Wolfram and lead, deleteriously produce considerable high-energied secondary gamma radiation upon interaction with neutrons their desirability is'largely restricted to special shielding application where the encountered radiation spectra have ah unusually high proportion of fast neutrons and/or garnmas to thermal neutrons-say at least 5' to 10 times as great as that designated in Table 1 supra. In such special applications, the significance of the efficacy of these high-atomic-weight elements in beneficially attenuating fast neutron and gamma radiation tends to outweigh that of its deleterious secondary gamma production, from an overall efficiency standpoint.

For the metal sheathing, the same general properties: malleability, neutron moderating efiicacy, and lack of secondary gamma radiation production, are particularly desirable. In addition, since the primary function of the sheathing is to protect and preserve disposition and integrity of the boron-material-containing matrix, it is also highly important that the sheathing metal should readily bond metallurgically with the metal employed to constitute the continuous matrix, and should have a high degree of resistance to whatever corroding and eroding conditions the plate maybe subjected to it its intended application. In instances where the continuous matrix is constituted of aluminum, and the plate is subjected merely to continuous contact with a relatively slowly moving stream of air or water, sheet aluminum has proven entirely satisfactory as the protective covering.

- Where more adverse corrosion and erosion conditions,

such as contact with aqueous solutions of strong acids and salts, boiling water, condensing vapors, and the like, stain ss steel sheathing is particularly preferable.

Concerning the proportions of boron material to malleable metal in the continuous matrix, it is desirable, from the standpoint of thermal-neutron-absorption effectiveness per unit thickness of plate, that the proportion of the boron material be as high as practicable. In the case of boron carbide particles and aluminum, about 50% boron carbide represents the maximum volumetric proportion that will still afford substantial continuity of the aluminum matrix; however, at such a high 134C proportion, the resulting mass is quite rigid, and is therefore somewhat difiicult to hot-roll or to be curved into desired contours. Where such mechanical operations are to be employed, it is desir able to employ somewhat lower boron carbide proportions, whence the diihculties are largely obviated; 35% 84C and 65% aluminum is the apparent volumetric optimum for affording good mechanical workability, together with a reasonably high boron density. Again, in the interest of thinness of the resulting plate, it is normally desirable that the metal sheathing be as thin as is consistent with effective protection of the boron-containing matrix. For B4.C-Al matrices of one-quarter inch thickness to be subjected merely to slow cooling-air streams, 0.02 inch aluminum sheathing on each side has been found fully adequate.

in further accordance with the present invention, a new and improved method, particularly eihcacious for the preparation of the present article, comprises: introducing, with stirring, the particles of refractory material substantially constituted of boron into a melt of the malleable metal selected for constituting the continuous matrix; cooling the resulting mixture; sandwiching the resulting continuous matrix of malleable metal having particles of the boron-containing refractory material uniformly dispersed therein between sheets of a malleable metal, and thereupon contemporaneously heating and rolling the resulting sandwich to reduce its thickness and simultaneously bond metallurgically said sheets to the continuous matrix of malleable metal. The malleable metal selected for constituting the continuous matrix should, of course, have a melting point substantially below that of the boron refractoryempl'oyed. ,For the preferred aluminum sheathed stirring. Since the boron carbide is not BiC-l-fAl plate, this procedure involves maintaining a melt of aluminum somewhat above its melting point (1220 F.)'-preferably about 13.004350 F.and thereupon slowly adding comminuted boron carbide thereto, while readily wet by the molten aluminum, maintaining the melt at a temperature well above the meltingpoint andmild stirring have been found important toward avoiding a tendency toward a crumbly and incohesive mass upon solidification. In this connection, it has been found that firmness and strength of the resulting boron-containing matrix is particularly favored by initially mixing the powdered boron carbide with about half of the aluminum in powdered form, and then adding the mixed powder slowly, with stirring, to the remainder of the aluminum in molten form; in practice, preheating the mixed powders to about 1000 F. before addition to the melt has proven to be a practical convenience. Alternatively, admixing a small amount (2-3%) boron oxide (B203) with the comminuted boron carbide before introducing it into the aluminum melt has also proven effective in promoting firmness and strength of the formed matrix. Upon cooling, the resulting ingot, about four or five times as thick as the desired ultimate plate, is sandwiched between a pair of aluminum sheets, also about four or five times as thick as the desired final sheathing, whereupon the sandwich is heated to a temperature of the order of 1000 to 1050 F. and hot rolled to the desired size. Such rolling results in a strong metallurgical bond between the aluminum sheathing and the continuous aluminum matrix, and eliminates virtually all porosity in the plate. Optionally, to obtain an especially strong metallurgical bond, the cooled ingot may be metal sprayed with aluminum, to cover all exposed boron carbide surfaces in the ingot, before sandwiching. As a practical matter, it is preferred to avoid rough, uneven edges of the final rolled plate, by initially placing the ingot, cast in the form of a rectangular slab, into a rectangular aluminum frame, close-fitting around the narrow edges of the slab, and thereupon sandwiching the so framed ingot between aluminum plates slightly larger than the frame; upon rolling, a plate is obtained having smooth, straight, narrow margins of pure aluminum, which are readily trimmed off before application to shielding.

Applicants plate, and method for preparing same, have general features making them singularly efiicacious for providing dependable neutron and reactor shielding. The continuous matrix serves to retain each particle of boron material positively fixed in place; hence, there is no reasonable risk of shifting or loss of components of the medium such that some areas of the shield might become hazardously inefiective-as would otherwise obtain, say, were particles of boron carbide and aluminum simply mixed and disposed in a thin container. Furthermore, particularly Where the boron is in carbide form and the matrix and sheathing metal is aluminum, the carbon and aluminum serve to decelerate faster neutrons to the thermal energy range, where they are most effectively absorbed by the boron, resulting in substantial enhancement in the overall shielding efiicctiveness of the medium. Casting in ingot form is singularly effective in providing a unitary continuous metallic matrix structure, while fully avoiding the difdculties of crumbled. matrix, loose boron material, no bonding of the sheathing, blisters caused by trapped air and by internal generation of gases through nuclear reaction, and loss of boron material upon machining, encountered in fruitless attempts to produce the present medium via hot rolling a mixture of boron carbide and aluminum powders enveloped in aluminum sheet. By virtue of comprising a continuous metallic phase, applicants plates have high thermal conductivity, which advantageously facilitates the escape of the considerable quantities of heat generated as a reaction to neutron capturea quality fundamentally lacking in loose powder systems. Moreover, being of rigid metal plate structure, applicants medium is particularly useful in applications demanding thin sections of shielding adapted to ready and rapidmechanical motion, such as shutters of neutron shielding material for intermittently barring, either partially or corn pletely, neutron flux from entering certain regions of a reactor, or for serving as a valve means for closing olf collimated thermal neutron beams purposely permitted to escape from the-reactor through void-channels through the shield provided therefor. For such applications, plates of the present medium quite simply may have the necessary hinges, trnnnions, kinematic linkages, and the like, Welded or bolted directly to them without special supporting or containing apparatus being needed.

Further illustration of the quantitative aspects and preferred compositions and procedures of the present shielding medium and method of making same is provided in the following specific example. In the example neutron shielding plate is produced in accordance with the present invention, employing specific composition and procedures particularly preferred for providing a layer of thermal neutron shielding at the inner face of a structural concrete shield surrounding a reactor emitting a radiation spectrum substantially of the order of that outlined in Table 1 supra.

EXAMPLE 825 grams of 28 aluminum pig were placed in a graphite crucible having dimensions of 7 inches high, 12 inches long, and 1% inches wide, with A inch taper, and were heated to, and maintained at, a temperatureapproximately l230 F.barely above the melting point of aluminum (1220 F.). A mixture of 1650 grams of comminuted boron carbide (20 to mesh) and 825 grams of aluminum powder (approximately 20 mesh) were slowly added to the crucible, and manually stirred into the molten aluminum with a inch steel rod, in order to wet and suspend the boron carbide particles in the aluminum melt. The crucible was thereupon cooled, and the resulting ingot was removed from the crucible; the ingot was noted to be somewhat porous (density: 2.25 g./cc.), but strong and homogeneous. The ingot was then metal-sprayed with a thin coat of aluminum to cover completely all exposed particles of boron carbide, and thereafter enclosed in a Vs inch wrapper of 28 aluminum sheet, which had been wire-brushed to remove excess oxide and dust. The aluminum-clad ingot was thereupon heated to approximately 1130 F., maintained at that temperature for approximately 1 hour, and thereupon rolled between steel rollers. Thickness was reduced approximately 10% upon each pass throughthe rolls; rolling was repeated until a plate of inch overall thickness was obtained, producing a finished sheet of 20 in. x 20 in. size. The material was noted to work heat appreciably during rolling, such that, with the reasonably rapid rolling pace employed, the temperature of the billet was sustained at approximately 1130 F. without the necessity of reheating between passes. Upon cooling, the somewhat uneven, slightly cracked edges were trimmed off straight and square, whereupon appropriate samples were cut from the plate for analysis and assessment of physical, chemical, mechanical, thermal, and nuclear properties, employing conventional analytical techniques. The nature and results of the analytical investigations are tabulated in Table 2 below.

Table 2.--Properties of Al-Clad B4C+Al plate 1. Composition: Thickness- Overall: in. Of cladding: 0.020 in. Volumetric proportion- 50% Etc; 50% A1 Chemical constitution Percent (wt) mgmnol/cm. g./cm. gJom.

2. Density:

2.53 g./cc. 0.09 lb./ in. 3 3% 1b./ft. 3 for A" sheet.

greases 3'. Strength:

(a) Tensile, 5,500 p. s. i.

(b) Elongation, 0.4%. v

(c) Shear, 8,237 p. s. i.

(d) Welded tensile specimens did not fail at weld; when welded with Al rod, B-iCflows into weld, maintaining the overall shielding etficacy.

(e) Strong, continuous bond of sheathingtoaluminum matrix.

. Thermal properties: a

(a) Conductivity: Somewhat better than steel.

(c) Melting point: Maintains mechanical strengthup to 1500 F. (800 above which oxida-- tion is appreciable. v

. Workability: Can be sheared, sawed, welded, punched,

drilled, tapped, rolled, hot-formed, and die-cast. 6. Nuclear properties:

Attenuation, by A" sheet, of incident thermal neutrons (i. e., neutrons of ca. 0.025 e. v. at room temperature) Incident Flux: Transmitted flux=1 10 :1.

The eminent suitability of the foregoing properties for thermal neutron shielding broadly, has prompted adoption of the important features of specific composition and procedure of the medium and method described in the example for mass production of such plates as a standard, universal neutron shielding medium. In production, ingots having dimensions 6 in. x 36 in. x 32 in., without taper, weighing ca. 450 pounds each, of composition substantially that in the example, are cast. The ingot is then fitted into a 1 inch thick, welded, 28 aluminum, open frame of internal dimensions of 36 in. x 32 in., and 6 in. deep. Each of-the two exposed faces of the framed ingot is covered by a wrapper sheet of 28 aluminum, /2 inch thick, of size slightly larger than the frame; the edges of the wrapper sheet are bent over the outer edges of the frame and tack welded to the outside of the frame, which has several vent holes drilled through it. Such wrapped ingots are hot-rolled into sheets 84 in. x 396 in.

x M4 in. on a large steel-plant rolling mill. From these,

Sheets of the present medium may also be disposed efiectively at various depths throughout conventional masonry shielding. Furthermore, taking additional advantage of its high thermal conductivity, a continuous, welded plate neutrons from the radiation spectrum. Furthermore,

aside from the improved shielding of nuclear reactors, the medium is well suited to the shielding of other intense thermal neutron sources, such as radium-beryllium mixtures; it is also ideal for the construction of sturdy metal shipping containers for transporting neutron-emitting materials, and for constituting protective shield discs mounted on the handles of radioactive-material handling tongs. Moreover, in view of its high neutron-absorption efiiciency, mechanical strength, and good thermal conductivity, the medium is eminently fitted to constitute control rods for regulating the neutron flux and fission rate in nuclear reactors. Various additional applications of the hereinbefore disclosed medium will become apparent to those skilled in the artl It is therefore to be understood that all matters contained in the above description and example are illustrativeonly and do not limit the scope of the present invention.

I What is claimed is:

1. An article of manufacture for improved shielding against thermal neutrons which comprises: a continuous matrix of a malleable metal; discontinuously and uniformly dispersed therein, particles of a solid refractory material substantially constituted of boron; and, containing said particle-bearing matrix, and substantially bonded metallurgically thereto, a sheathing of a malleable metal.

2. The articleof claim 1 wherein said malleable matrix metal is aluminum.

3. The article of claim 1 wherein said malleable matrix metal is zirconium.

4. The article of claim 1 wherein said solid refractory material substantially constituted of boron is boron carbide;

5. The article of claim 1 wherein said solid refractory material substantially constituted of boron is boron oxide.

6. The article of claim 1 wherein said solid refractory material substantially constituted of boron is elemental boron.

7. The article of claim 1 wherein said malleable sheathing metal is aluminum.

8. The article of claim 1 wherein said malleable sheathing metal is stainless steel.

9. The article of claim 1 wherein said malleable matrix metal is aluminum, and said solid refractory material substantially constituted of boron is boron carbide.

10. The article of manufacture of claim 1 wherein said malleable matrix metal is aluminum, said solid refractory material substantially constituted of boron is boron carbide, and the volumetric proportion of solid refractory material to malleable matrix metal is approximately 121.

11. The article of claim 1 wherein said malleable matrix metal is zirconium, and said solid refractory material substantially constituted of boron is boron carbide.

12. The article of claim 1 wherein said malleable matrix metal is aluminum, and said solid refractory material substantially constituted of boron is boron oxide.

13. The article of claim 1 wherein said malleable matrix metal is aluminum, said solid refractory material is boron carbide, and said malleable sheathing metal is aluminum. I

14. An improved method for preparing an article of manufacture for improved shielding against thermal neutrons comprising a continuous matrix of a malleable metal, particles of a solid refractory material substantially constituted of boron discontinuously and uniformly dispersed in said matrix, and, containing said particle-bearing matrix and substantially bonded metallurgically thereto, a sheathing of a malleable metal, which comprises: introducing, and stirring said particles into a melt of said malleable matrix metal having a melting point substantially below that of said refractory material; cooling the resulting mixture; sandwiching the resulting solid continuous matrix of malleable metal having particles of boroncontaining refractory material uniformly dispersed therein between sheets of said malleable sheathing metal; and thereupon contemporaneously heating and rolling the resulting sandwich to thereby reduce its thickness and simultaneously bond metallurgically said sheets to the continuous matrix of malleable metal.

15. The method of claim 14 wherein said malleable matrix metal is aluminum, said solid refractory material is boron carbide, and said malleable sheathing metal is aluminum.

16. The method of claim 14 wherein said malleable matrix metal is aluminum, said solid refractory material is 17. The method of claim-l4 .wherein said malleable matrix metal is aluminum, said solid refractory material is boron carbide, said malleable sheathing metal is aluminum, and said particles are introduced to the melt in the form of an admixture with aluminum powder.

18. The method of claim 14 wherein said malleable matrix metal is aluminum, said solid refractory material is boron carbide, said malleable sheathing metal is aluminum, and said particles are introduced to the melt in the form of an admixture, with aluminum powder, heated to a temperature of approximately 1000 F.

19. The method of claim 14 wherein said malleable matrix metal is aluminum, said solid refractory material is boron carbide, said malleable sheathing metal is aluminum, and said particles are introduced to the melt in the form of an admixture with a minor proportion of comminuted boron oxide.

20. The method of claim 14 wherein said malleable matrix metal is aluminum, said solid refractory material is. boron carbide, said malleable sheathing metal is aluminum, and said sandwich is maintained at a temperature of the order of 1000" to 1050 F. while rolling.

21. The method of claim 14 wherein said malleable matrix metal is aluminum, said solid refractory material is boron carbide, said malleable sheathing metal is aluminum, and said resulting solid continuous matrix of malleable metal having particles of refractory material dispersed therein is metal-sprayed with aluminum before said sand- Wiching. v

v 22. The method of claim 14 wherein said resulting solid continuous matrix is cast in the form of a substantially rectangular slab, the resulting slab is disposed in an open rectangular frame of malleable metal close-fitting around the narrow edges of said slab, and thereupon effecting said sandwiching, between sheets of malleable sheathing metal, of the so framed slab. v y

23. The method of claim 14 wherein said malleable matrix metal is aluminum, wherein said solid refractory material is boron-carbide, wherein said malleable sheathing metal is aluminum, and whereinsaid resulting solid continuous matrix is cast in the form of a substantially reetangular slab, the resulting slab is disposed in an open rectangular frame of aluminum close-fitting around the narrow edges of said slab, and thereupon is effected said sandwiching, between sheets of malleable sheathing metal, of the so framed slab.

No references cited. 

1. AN ARTICLE OF MANUFACTURE FOR IMPROVED SHIELDING AGAINST THERMAL NEUTRONS WHICH COMPRISES: A CONTINUOUS MATRIX OF A MALLEABLE METAL; DISCONTINUOUSLY AND UNIFORMLY DISPERSED THEREIN, PARTICLES OF A SOLID REFRACTORY MATERIAL SUBSTANTIALLY CONSTITUTED OF BORON; AND, CONTAINING SAID PARTICLE-BEARING MATRIX, AND SUBSTANTIALLY BONDED METALLURGICALLY THERETO, A SHEATHING OF A MALLEABLE METAL. 